Although formaldoxime, hereinafter sometimes referred to as "FOX-T", is the first and simplest aliphatic aldoxime in the aliphatic series, its manufacture embodies a series of problems which devolve from both the high reactivity of the raw material used in its manufacture, namely, formaldehyde and the tendency of the product formaldoxime monomer (FOX-M) to convert to a formaldoxime trimer (FOX-T). Specifically, formaldehyde exists as a monomer, as polymers and as a trimer, paraformaldehyde; and the product oxime exists not exclusively as the low boiling liquid monomer but also as a high melting solid polymer/trimer.
Formaldehyde exists as the following structures in solution in water: CH.sub.2 (OH).sub.2, methylene glycol; poly(oxymethylene glycols), HO(CH.sub.2 O).sub.n H; hemi formals of these glycols; HCHO; --(CH.sub.2 O).sub.n --; and a cyclic trimer as shown below: ##STR1##
The cyclic trimer may be isolated as a white crystalline powder by concentrating the aqueous solutions. This trimer is an article of commerce.
Generally, methanol is used, 10-15 w % in 35-40 w % aqueous solutions of formaldehyde (HCHO), to inhibit such polymer-trimer formation.
Likewise formaldoxime can exist as a water soluble monomer CH.sub.2 =NOH but will rapidly revert to the polymer ##STR2## wherein "n" has a variable value; or to the trimer ##STR3##
The variable value of "n" is known and discussed in the prior art and as such reference is made, for example to K. A. Jensen and A. Holm, Investigations of Formaldehyde Oxime, Its Polymers and Coordination Compounds, I, Mat. Fys. Med. 40:1, pp. 3-23 (1978), and E. A. Andersen and K. A. Jensen, Investigations of Formaldehyde Oxime, Its Polymers and Coordination Compounds, II, Mat. Fys. Med. 40:2, pp. 3-24 (1978).
The trimer is a high melting solid (MP145.degree.-156.degree. C.); the exact melting point depending on polymer-trimer ratios in the mixture which are believed to exist but which are not readily determinable. The monomer, on the other hand, is a liquid, BP 89.degree. C.
Literature source preparation of the trimer, FOX-T, has involved the addition of formaldehyde to a pre-neutralized aqueous solution of hydroxylamine salt followed by prolonged low temperature storage of the solution to allow precipitation of the formaldoxime trimer which is then recovered by filtration. It has been found that this method results in the formation of a very fine precipiate of FOX-T which adversely blinds the filter device such that recovery is very slow. The solid also occludes large amounts of the salt of the acid entity associated with the hydroxylamine salt which is difficult to remove from the FOX-T without considerable washing and some loss thereof due to some degree of reversion of FOX-T to the formaldoxime monomer FOX-M. There is also direct yield loss due to the presence of the monomer and the reaction of the monomer which has not gone to the trimer in the mother liquor from the primary recovery filtration and the wash.
In the reaction in water, there apparently exists an equilibrium between the trimer and monomer. The formaldoxime monomer stays as the monomer in dilute solution but when formaldoxime monomer reaches 15-20 weight percent, the trimer will start to form. The rate and extent of the trimer formation seem to depend on temperature/pH/time. The lower the temperature, the longer the reaction time and the more the trimer which forms. However, there is always some formaldoxime monomer formation. The higher the pH, the more the amount of monomer which forms.